The present invention relates generally to improvements in thermometers and, more particularly, to electronic thermometers for more rapidly obtaining accurate temperature measurements.
It is common practice in the medical field to determine the body temperature of a patient by means of a temperature sensitive device that not only measures the temperature but also displays that temperature. Such temperature measurements are taken routinely in hospitals and in doctors"" offices. One such device is a glass bulb thermometer incorporating a heat responsive mercury column that expands and contracts adjacent a calibrated temperature scale. Typically, the glass thermometer is inserted into the patient, allowed to remain inserted for a sufficient time interval to enable the temperature of the thermometer to stabilize at the body temperature of the patient, and subsequently removed for reading by medical personnel. This time interval is usually on the order of 3 to 8 minutes.
The conventional temperature measurement procedure using a glass bulb thermometer or the like is prone to a number of significant deficiencies. Temperature measurement is rather slow and, for patients who cannot be relied upon (by virtue of age or infirmity) to properly retain the thermometer for the necessary period of insertion in the body, may necessitate the physical presence of medical personnel during the relatively long measurement cycle, thus diverting their attention from other duties. Furthermore, glass bulb thermometers are not as easy to read and, hence, measurements are more susceptible to human error, particularly when the reading is made under poor lighting conditions or when read by harried personnel.
Various attempts have been made to minimize or eliminate these deficiencies of the glass bulb thermometer by using temperature sensing probes that are designed to operate in conjunction with direct-reading electrical thermometer instrumentation. In one such approach, an electric temperature sensitive device, such as a thermistor, is mounted at the end of a probe and inserted into the patient. The change in voltage or current of the device, depending on the particular implementation, is monitored and when that output signal stabilizes, a temperature is displayed in digital format. This is commonly referred to as the xe2x80x9cdirect readingxe2x80x9d approach and it reduces the possibility of error by misreading the measured temperature. This approach has provided a significant contribution to the technology of temperature measurement.
An inherent characteristic of electronic thermometers is that they do not instantaneously measure the temperature of the site to which they are applied. It may take a substantial period of time before the temperature sensitive device stabilizes at the temperature of the site and the temperature indicated by the thermometer is representative of the actual temperature of the body or site measured. This lag is caused by the various components of the measurement system that impede heat flow from the surface of the body or site to the temperature sensor. Some of the components are the sensor tip, the tissue of the body, and any hygienic covering applied to the sensor tip to prevent contamination between measurement subjects.
One attempt to shorten the amount of time required to obtain a temperature reading of a subject involves the use of a temperature sensitive electronic probe coupled with prediction or estimation circuitry or programming to provide a digital display of the patient""s temperature before the probe has reached equilibrium with the patient. With this approach, assuming the patient""s temperature is not significantly changing during the measurement period or cycle, the temperature that will prevail upon thermal stabilization of the electronic thermometer with the patient is predicted from measured temperatures and is displayed before thermal stabilization is attained. Typically, prediction of temperature is performed by monitoring the measured temperature over a period of time as well as the rate of change thereof, and processing these two variables to predict the patient""s temperature.
With an electronic thermometer that operates by predicting the final, steady state temperature, an advantage is that the temperature measurement is completed before thermal stabilization is attained, thereby reducing the time required for measurement. This would lessen the risk that the patient would not hold the probe in the correct position for the entire measurement period and requires less time of the attending medical personnel. Another advantage is that because body temperature is dynamic and may significantly change during the five minute interval typically associated with traditional mercury glass thermometer measurements, a rapid determination offers more timely diagnostic information. In addition, the accuracy with which the temperature is predicted improves markedly as the processing and analysis of the data are more accurately performed. This approach has also contributed significantly to the advancement of temperature measurement technology.
Electronic thermometers using predictive-type processing and temperature determination may include a thermistor as a temperature-responsive transducer. The thermistor approaches its final steady state temperature asymptotically with the last increments of temperature change occurring very slowly, whereas the major portion of the temperature change occurs relatively rapidly. Prior attempts have been made to monitor that initial, more rapid temperature change, extract data from that change, and predict the final temperature at which the thermistor will stabilize and therefore, determine the actual temperature of the tissue that is contacting the thermistor long before the thermistor actually stabilizes at the tissue temperature.
A prior approach used to more rapidly predict the tissue temperature prior to the thermistor reaching equilibrium with that tissue is the sampling of data points of the thermistor early in its response and from those data points, predicting a curve shape of the thermistor""s response. From that curve shape, an asymptote of that curve and thus the stabilization, or steady state, temperature can be predicted. To illustrate these concepts through an example of a simpler system, consider the heat transfer physics associated with two bodies of unequal temperature, one having a large thermal mass and the other having a small thermal mass, placed in contact with each other at time t=0. As time progresses, the temperature of the small thermal mass and the large thermal mass equilibrate to a temperature referred to as the stabilization temperature. The equation describing this process is as follows:
T(t)=TR+(TFxe2x88x92TR) (1xe2x88x92exe2x88x92(t/xcfx84))=TFxe2x88x92(TFxe2x88x92TR)exe2x88x92(t/xcfx84)xe2x80x83xe2x80x83(Eq. 1)
where:
T(t) is the temperature of the smaller body as a function of time,
TR is the initial temperature of the smaller body,
TF is the actual, steady state temperature of the system,
t is time, and
xcfx84 is the time constant of the system.
From this relationship, when T is known at two points in time, for example T1 at time t1 and T2 at time t2, the stabilization temperature TF can be predicted through application of Equation 2 below.                                           T            F                    =                                                                      T                  2                                -                                                      T                    1                                    ⁢                                      ⅇ                                          -                                              xe2x80x83                                            ⁢                                                                                                    t                            2                                                    -                                                      t                            1                                                                          τ                                                                                                                        1                -                                  ⅇ                                      -                                                                                            t                          2                                                -                                                  t                          1                                                                    τ                                                                                            =                                                                                T                    2                                    ⁢                                      ⅇ                                                                  t                        2                                            τ                                                                      -                                                      T                    1                                    ⁢                                      ⅇ                                                                  t                        1                                            τ                                                                                                                    ⅇ                                                            t                      2                                        τ                                                  -                                  ⅇ                                                            t                      1                                        τ                                                                                                            (Eq.  2)                      ⁢      xe2x80x83  
Further, for a simple first order heat transfer system of the type described by Equation 1, it can be shown that the natural logarithm of the first time derivative of the temperature is a straight line with slope equal to xe2x88x921/xcfx84 as follows:                               ln          ⁢                      xe2x80x83                    ⁢                      (                                          ⅆ                T                                            ⅆ                t                                      )                          =                  K          -                      t            τ                                              (Eq.  3.1)            
and also:
TF=T(t)+xcfx84Txe2x80x2(t)xe2x80x83xe2x80x83(Eq. 3.2)
where:                     τ        =                  -                                                    T                xe2x80x2                            ⁡                              (                t                )                                                                    T                xe2x80x3                            ⁡                              (                t                )                                                                        (Eq.  3.3)            
where:
K=a constant dependent upon TR, TF, and xcfx84,
Txe2x80x2=first derivative
Txe2x80x3=second derivative
Prior techniques have attempted to apply these simple first order relationships through the use of thermistor time constants established by the thermistor manufacturer. However, these techniques have failed to recognize that the temperature response curve cannot be modeled as first order and is to a great extent affected by factors not reflected by the thermistor""s time constant. When the thermometer is placed in contact with body tissue, such as a person""s mouth for example, the response curve depends on the physical placement of the probe in relation to that tissue, on the heat transfer characteristics of the particular tissue, and on the hygienic cover that separates the probe from the tissue. All of these factors contribute to the heat flow characteristics of the measurement system and they are not represented in the factory-supplied time constant of the thermistor alone. Moreover, the factors described above impede the flow of heat in series and with different resistance characteristics, thus causing an overall time response behavior that is not that of a first order system.
While electronic thermometers and prior predictive techniques have advanced the art of electronic thermometry, a need still exists for an electronic thermometer that can predict a stabilization temperature at an early stage of the measurement process and thereby shorten the amount of time taken to obtain a final temperature reading. Additionally, a need exists for a thermometer having an algorithm that can be computed in readily available, relatively simple, relatively inexpensive circuitry or processors. The invention fulfills these needs and others.
Briefly and in general terms, the present invention is directed to providing an electronic thermometer and a method for determining the temperature of an object or biological subject by predicting the steady state temperature at an early stage of the measurement process. The thermometer and method of the present invention relate certain variables determined from an early portion of the temperature rise curve to the predicted steady state temperature so that the predictive process requires a reduced process of data acquisition and a shortened data processing time while yielding accurate approximations of the stabilization temperature.
Thus, briefly and in general terms, in one aspect of the present invention is directed to a thermometer incorporating a temperature sensor, a processor for predicting an object""s temperature based on the average value, slope, and curvature of the initial reading of the object""s temperature, and a display for displaying the predicted temperature.
In a more detailed aspect, the processor comprises a finite impulse response filter connected so as to sample the temperature signal a plurality of times to calculate an estimate of the temperature of the subject and provide an estimated final temperature signal and a display connected with the processor to receive and display the estimated final temperature signal. In yet further detail, the finite impulse response filter takes a linear combination of a plurality of samples in calculating the estimate of the temperature of the subject. In another aspect, the processor adds an offset coefficient based on ambient temperature to the estimate of the temperature provided by the finite impulse response filter in providing an estimated final temperature signal.
In another aspect, a thermometer for determining the temperature of an object is provided and comprises a sensor that provides a time varying temperature signal in response to sensing the temperature of the object, a processor that receives the temperature signal, samples the temperature signal over a time frame, determines the average value, the first derivative, and the second derivative of the signal over the time frame, combines the average value, first derivative, and second derivative, and thus calculates an estimate of the temperature of the object. In a more detailed aspect, the processor applies a weighting factor to each of the average value, the first derivative, and the second derivative of the signal, and further adds an offset factor selected in accordance with the ambient temperature, to calculate a prediction of the temperature of the object.
In further detailed aspects, the processor further comprises finite impulse response filters to calculate the average value, slope, and curvature of the temperature data. In a more detailed aspect, the processor continues to sample the signal and calculate a new prediction for the temperature of the object with each new temperature data value sampled.
In another detailed aspect, the processor monitors a predetermined number of the last predicted temperatures and calculates the final predicted temperature of the object based on an average of these last predicted temperatures. In yet another detailed aspect, the processor calculates the final predicted temperature when certain selected conditions have been met. In a still further aspect, the selected conditions include a predetermined time period that must lapse after the sensor has been in contact with the object, predetermined threshold values that the first derivative and the second derivative must reach, and a maximum difference between any two of a predetermined number of the last temperature estimates that must be less than a predetermined threshold.
These and other features and advantages of the present invention will become apparent from the following more detailed description, when taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.